A magnetic drive system of the type indicated above is known and is used, for example, for driving rotary pumps and, more specifically, centrifugal pumps, for example, for use in the presence of fluids which are hazardous because they are corrosive or highly polluting.
In the situation specified above, the driving element is generally the output shaft of an electric motor, although rotary motors of other types may be used, and the driven element, naturally, is part of an impeller of the centrifugal pump.
Pumps which use drive systems of this type are known as so-called “seal-less” pumps, that is, pumps in which the drive is transmitted from the driving element to the driven element, and hence to the impeller, without the interposition of seals.
Drive systems of this type, and consequently pumps of this type, are used mainly in chemical plants in which one of the most important requirements relates to the limitation of emissions and particularly of possible emissions from the seal of the shaft. In fact, as is known, in drive systems of the type indicated above, the driving element is not in contact with the driven element since a containment element, commonly known as a bell, is interposed between the two and, together with the basic structure, encapsulates the environment containing the impeller and confines the liquid inside the pump, thus preventing the risk of emissions.
As is known, a drive system of this type has a first ring constituted by a plurality of magnets fixed to the driven element and having alternating opposite polarities and a second ring, constituted by a plurality of magnets fixed to the driving element and having alternating opposite polarities. The two rings are coaxial and face one another. Naturally, the driving element and the driven element are also arranged in a manner such that each magnet of the driving element faces a magnet of the driven element having the opposite polarity. As a result, a rotation of the drive shaft, and hence of the driving element, tends to move magnets of the same polarity and disposed on the two elements, respectively, towards one another. The driven element is consequently urged to rotate as a result of the repulsion between the polarities of the same sign belonging to the driving element and to the driven element, respectively.
In the case of the impeller, that is, of the driven element, a known drive system has a sleeve of magnetic material, for example, ferrite steel, on which the magnets are positioned at regular intervals and glued so as to close the magnetic flux. The sleeve to which the magnets are glued becomes an insert to be incorporated in a casting of plastics material, for example, polypropylene.
In the case of the driving element, which generally represents the outer portion of the drive system, use is also made of a sleeve on which the magnets are positioned and glued at regular intervals. The sleeve can then be covered with a protective layer or incorporated in a moulding.
In both cases, the mounting of the magnets requires them to be fitted on a sleeve of magnetic material to which they adhere by magnetic attraction and to which they are also fixed by gluing.
The drive systems described above, and consequently the pumps which use them, have considerable disadvantages which can be summarized as follows. In the first place, the correct positioning and fixing of the magnets involves great skill on the part of the operator who performs the assembly since the magnetic forces in play are quite large to the extent that, during the last portion of its movement towards the sleeve, the magnet is pulled against the sleeve. Since the operator loses control of the magnet during the last portion of its movement, it is difficult to achieve a high degree of accuracy in the positioning of the magnets on the sleeve and, in the event of incorrect positioning, it is practically impossible to detach a magnet in order to reposition it. Moreover, the force with which the magnet is drawn against the sleeve is often of a magnitude such as to damage the magnet and in particular its edges and to introduce risks to the safety of the operator's fingers.
In the second place, the step of the gluing of the magnets slows down production and does not offer a secure restraint, particularly in relation to the centrifugal forces which are exerted on the magnets in operation, particularly in the case of the impeller, and hence of the driven element.
These disadvantages are aggravated as the number of magnets increases and, in particular, when there are several rings of magnets to be fixed both to the driven element and to the driving element.
In addition to the foregoing disadvantages, there is the fact, with regard to the shape of the magnets in particular, that there are sharp edges which may give rise to dangers to the operator.
Finally, it should be borne in mind that the number of magnets affects the transmission which the system can achieve. As a result, the number of magnets to be used depends on the process conditions to be established and, for known drive systems, it is therefore necessary to maintain a store with a series of sleeves having different numbers of glued magnets.
The problem upon which the present invention is based is that of proposing a mechanical drive system operating by magnetic force, a magnet for a mechanical drive system operating by magnetic force, a method of producing a mechanical drive system operating by magnetic force, and a pump, which have structural and functional characteristics such as to satisfy the above-mentioned requirements and, at the same time, to overcome the disadvantages mentioned with reference to the prior art.